fix: corrected threshold for luminance (#472)

The correct threshold is 0.04045, as recited in the official IEC standard for sRGB. And while it was incorrect (0.03928) for a long time in WCAG2, it was corrected by the W3 AGWG in May 2021. 

Current WCAG 2.1 and 2.2 reflect the correct 0.04045 threshold.

Co-authored-by: Rico Kahler <[email protected]>

src/getLuminance.ts

@@ -10,7 +10,7 @@ function getLuminance(color: string): number {
 
   function f(x: number) {
     const channel = x / 255;
-    return channel <= 0.03928
+    return channel <= 0.04045
       ? channel / 12.92
       : Math.pow(((channel + 0.055) / 1.055), 2.4);
   }